The invention relates to a force-fit connection between prosthesis components of joint prostheses. Components of joint prostheses in accordance with the preamble of the first claim.
Joint prostheses, in which one joint partner is formed as a socket and the other joint partner is formed as a spherical head which is rotatably mounted in the socket, are known in particular as shoulder-joint and hip-joint prostheses. These prostheses, as a rule, are built up in a modular fashion. Hip-joint endoprostheses, for example, consist of the socket, which is inserted into the hip bone, and of the shaft, which is inserted into the femur. The socket consists as a rule of a metallic outer shell into which a shell insert made of ceramic material or a biocompatible plastics material is inserted. A force-fit connection is used here as the connecting technique. A force-fit connection is known, for example, from DE 196 11 248 A1. The shaft has a peg, the so-called cone, onto which the spherical head is slipped. In the case of the modularly constructed endoprostheses, implant components are connected together that are made of different materials and are of different sizes for the purposes of adaptation to the physique of the patient. For example, spherical heads made of a cobalt-chromium alloy or made of an aluminium-oxide ceramic material are slipped onto a cone made of titanium. The force-fit connection, in particular conical jamming, is also used here as the connecting technique between metallic or ceramic spherical heads and the cone. In this connection, the spherical head which has a conical bore is placed upon the cone. After the spherical head has been slipped onto the cone, fixing is effected by hitting the spherical head.
In the case of conical jamming, the joint surfaces of the cone and the bore in the spherical head must be worked in an extremely precise manner, because otherwise the durability of the conical fit is jeopardized as a result of uneven introduction of force into the spherical head or the cone respectively. If the spherical head is made of ceramic material, stress peaks in the material can result in the formation of cracks and, in the worst case, the destruction of the spherical head. In order to make the conical fit more reliable, coupling elements have therefore been proposed and these are to compensate for dimensional inaccuracies and are to improve the durability of the force-connection. Such a coupling element is known from DE 40 08 563 A1. It is a sleeve made of metal, preferably of titanium, which has a predetermined degree of roughness on its inner and outer faces. After the spherical head has been placed on the cone and after the roughness has been lost, such a sleeve is so rigid, like the cone itself, with the disadvantages set forth above.
The object of the present invention is to propose, for joint prostheses, an improved force-fit connection between the spherical head and the cone.
The object is achieved by using a coupling element for rendering uniform the transfer of force between the cone and the spherical head, the elasticity and damping properties of the coupling element being predetermined by its porosity and the structure of its surface.
As a result of an aimed for configuration of the surface structure and the internal structure of the coupling element, in particular its porosity, in accordance with the invention, its elasticity and damping properties and thus the introduction of force into the bodies that are coupled therewith are predetermined. As a result of the force-fit connection in accordance with the invention having a porous coupling element with a structured surface it is also possible to exchange damaged spherical heads without having to remove the shaft, even if the state of the surface of the cone would no longer permit the spherical heads to be placed directly thereon.
One embodiment of the porous coupling elements can be produced as a wound body. As a result of controlled deposition of the threads on a body, which has at least approximately the shape that the coupling element is to obtain, it is possible to control the porosity of the wound body and its surface structure. In the case of cylindrically or conically shaped wound bodies, which can be wound, for example, on tubes on spooling machines, the porosity can be predetermined by adjusting the intersection angle, the spacing of the threads deposited side by side (thread traverse) and also the thread tension during the winding process.
Instead of having a wound body, the coupling element can also have a structure that is like that of a woven fabric. It is possible to influence the porosity and the nature of the surface, the roughness, by means of the mode of weaving, that is, the distance of the threads from each other and their interlacing. In addition, the structure of a coupling element consisting of a plurality of layers of woven fabric affords the possibility of influencing the porosity and thus its deformability.
The surface structure in the case of the wound bodies and in the case of the woven fabrics is substantially determined by the diameter of the threads, the shape of the threadsxe2x80x94flat, round or twistedxe2x80x94and also the deposition or interlacing of the threads. Owing to the fact that, both in the case of the wound body and also in the case of a structure like that of a woven fabric, the threads can be worked with variable spacing from each other, elevations develop at the points of intersection of the threads on the surface of the wound bodies or the woven fabric, whilst the pores develop on account of the intervals between the threads. The points of intersection and the intervals between the threads therefore influence on the surface the structure, the roughness, of the coupling element.
Although the coupling element between the spherical head and the cone is substantially screened from the bone and the body tissue, the possibility of interaction between the material of the coupling element and the body tissue does nevertheless exist, on account of the body fluid. For this reason, the threads of the wound bodies or the woven fabrics are produced from a biocompatible material. All the materials which have already been considered as being biocompatible in prosthetics are suitable as materials. The threads can therefore be made, for example, of metal or a metal alloy, such as, for example, chromium, tungsten- chromium, cobalt-chromium, and titanium. Threads made of carbon that have a particularly high level of tensile strength and, moreover, consist of an element that is present in the body itself have proved to be particularly effective. In particular, by using threads made of carbon it is possible to configure a coupling element in such a way that when loaded it does not deform in the direction of its surface extension.
In order to facilitate the production of a coupling element in the correspondingly desired form, it is advantageous if the threads or the layers of threads are fixed in their position by means of a fixing agent. Epoxy resins can be used, for example, as fixing agents. It is also essential here that the fixing agent be made of a bioinert or biocompatible material.
In a further embodiment, besides wound bodies and woven fabrics, it is also possible to produce coupling elements as sintered bodies or sponge bodies. For example, coupling elements can be sintered in any desired shape from spheres or grains of a biocompatible material, in particular from the known biocompatible metals already listed above, in which case the pore size within the sintered body and also its surface structure are simultaneously determined by the sphere size or grain size. In the case of sponge bodies, which can be produced, for example, by gassing molten metals, it is possible to influence the pore content and the pore size by appropriate supply of the gassing agent.
Depending on the anticipated loading and size of the endoprosthesis, the coupling element for a spherical head can have a wall thickness between 0.3 mm and approximately 2 mm. A range between 0.5 mm and 1 mm is preferred. The wall thickness of the coupling element is taken into consideration when the conical bore is introduced into the spherical head. The proportion of pores of the coupling element, matched to the desired elasticity and damping, can be adjusted to a proportion between 20% and 90%, with a proportion of pores of approximately 40% being regarded as optimum. The higher the proportion of pores is, the more elastically the coupling element behaves.
The roughness of the surface of the coupling element also exerts an influence on the elasticity and damping. With a comparatively high level of surface roughness, the deformability of the surface increases and, as a result, the elasticity of the coupling element and its damping properties increase. The surface roughness is likewise matched to the endoprosthesis and, there can be a roughness-height Ra lying, between 2 xcexcm and 300 xcexcm. A roughness-height of approximately Ra=60 xcexcm has proved to be advantageous.
The surface structure, that is, the roughness-height, as set forth above, is influenced by the winding processes or the weaving processes or by the sintering technique or the pore production. In particular, in the case of the coupling elements that are sintered from metallic materials and in the case of the sponge bodies, it is possible to influence the structure and thus the roughness further in a controlled manner by means of subsequent treatment of the surface, for example by grinding, sand-blasting or over-twisting.